FBAR devices that incorporate one or more film bulk acoustic resonators (FBARs) form part of an ever-widening variety of electronic products, especially wireless products. For example, modern cellular telephones incorporate a duplexer in which each of the band-pass filters includes a ladder circuit in which each element of the ladder circuit is an FBAR. A duplexer incorporating FBARs is disclosed by Bradley et al. in U.S. Pat. No. 6,262,637 entitled Duplexer Incorporating Thin-film Bulk Acoustic Resonators (FBARs), assigned to the assignee of this disclosure and incorporated by reference. Such duplexer is composed of a transmitter band-pass filter and a receiver band-pass filter. The transmitter band-pass filter is connected between the output of the transmitter and the antenna. The receiver band-pass filter is connected in series with 90° phase-shifter between the antenna and the input of the receiver. The center frequencies of the pass-bands of the transmitter band-pass filter and the receiver band-pass filter are offset from one another. Ladder filters based on FBARs are also used in other applications.
An FBAR is composed of opposed planar electrodes and a piezoelectric element between the electrodes. The FBAR is suspended over a cavity defined in a substrate, which allows the FBAR to resonate mechanically in response to an electrical signal applied between the electrodes.
U.S. patent application Ser. No. 10/699,289 of Larson III, assigned to the assignee of this disclosure and incorporated by reference, discloses a band-pass filter that incorporates a decoupled stacked bulk acoustic resonator (DSBAR). The DSBAR is composed of a lower FBAR, an upper FBAR stacked on lower FBAR and an acoustic decoupler between the FBARs. Each of the FBARs is structured as described above. An electrical input signal is applied between electrodes of the one of the FBARs and the other of the FBARs provides a band-pass filtered electrical output signal between its electrodes.
U.S. patent application Ser. No. 10/699,481 of Larson III et al., assigned to the assignee of this disclosure and incorporated by reference, discloses a film acoustically-coupled transformer (FACT) composed of two decoupled stacked bulk acoustic resonators (DSBARs). A first electrical circuit interconnects the lower FBARs of the DSBARs in series or in parallel. A second electrical circuit interconnects the upper FBARs of the DSBARs in series or in parallel. Balanced or unbalanced FACT embodiments having impedance transformation ratios of 1:1 or 1:4 can be obtained, depending on the configurations of the electrical circuits. Such FACTs also provide galvanic isolation between the first electrical circuit and the second electrical circuit.
FBARs and devices, such as ladder filters, DSBARs, FACTs and band-pass filters, incorporating one or more FBARs will be referred to generically in this disclosure as FBAR devices.
Most FBAR devices have a frequency response having a band pass characteristic characterized by a center frequency. The constituent FBARs have a frequency response characteristic characterized by a resonant frequency. In practical embodiments of current FBAR devices in which the material of the piezoelectric element is aluminum nitride (AlN) and the material of the electrodes is molybdenum (Mo), the resonant frequency of each FBAR has a temperature coefficient that ranges from about −20 ppm/° C. to about −35 ppm/° C. The temperature coefficient of the resonant frequency reduces the temperature range over which the FBAR device incorporating the FBAR can meet its pass bandwidth specification. The temperature coefficient of the resonant frequency additionally reduces manufacturing yield, because the pass bandwidth limits to which the FBAR devices are tested have to be inset from the pass bandwidth specification to ensure that the FBAR device will meet the pass bandwidth specification over its entire operating temperature range.
U.S. patent application Ser. No. 10/977,398 of Larson III et al., assigned to the assignee of this disclosure and incorporated by reference, discloses FBAR devices that incorporate a temperature compensating element that effectively reduces the temperature coefficient of the pass bandwidth of the FBAR device. However, the temperature compensating elements disclosed in the application are composed of materials not normally used in the semiconductor fabrication-based processes typically used to fabricate FBAR devices. The need to use unique processing to form the temperature-compensating element is economically disadvantageous.
In Temperature Compensated Bulk Acoustic Thin Film Resonator, 2000 IEE ULTRASONICS SYMPOSIUM, 855-858, Lakin et al. disclose that silicon dioxide (SiO2) has a positive temperature coefficient in the temperature range 20° C.-80° C. The positive temperature coefficient of SiO2 is opposite that of aluminum nitride and molybdenum, which are materials commonly used in FBARs. An SiO2 temperature compensating element reduces the temperature coefficient of the resonant frequency of the FBAR to about one half of that of an uncompensated FBAR. Using SiO2 as the material of the compensating element is attractive for the additional reason that SiO2 deposition and patterning is a well-established, standard semiconductor fabrication process.
Lakin et al. disclose the use of SiO2 in the context of a solidly-mounted resonator. However, suspended FBAR devices, in which a suspended FBAR stack comprising the FBAR(s) is suspended over a shallow cavity defined in a substrate, typically have better performance than solidly-mounted resonators. The suspended FBAR stack is typically fabricated on the surface of a support layer of sacrificial material, typically phosphosilicate glass (PSG), that fills the cavity. After the FBAR stack has been fabricated, a release etch is performed to remove the sacrificial material from under the FBAR stack. This leaves the FBAR stack suspended over the cavity. The release etch typically uses dilute hydrofluoric acid (HF). However, HF additionally aggressively attacks SiO2. Accordingly, a temperature compensating element composed of a layer of SiO2 is incompatible with the release etch performed after the FBAR stack has been fabricated. What is needed, therefore, is a way of fabricating a suspended FBAR device in which the FBAR stack comprises a material, such as SiO2, that is incompatible with the release etch performed after the suspended FBAR stack has been fabricated.
Many other types of suspended devices, in which a suspended device structure is suspended over a shallow cavity, are fabricated on a support layer that is removed from under the device structure in a release etch performed after the device structure has been fabricated. Performing the release etch after the device structure has been fabricated limits the materials that can be used in the device structure to those that are compatible with the release etchant. This limitation can be problematical. What is additionally needed, therefore, is a way of fabricating a suspended device in which the device structure comprises a material that is incompatible with the release etchant.